1. Field of the Invention
The present invention relates a sheet feeding apparatus for separating and feeding a sheet one by one by the cooperation of a feeding means such as rubber roller or rubber belt with a frictional separating means such as a rubber sheet embedding cork therein, which is urged against the feeding means.
2. Related Background Art
As shown in FIG. 11, a conventional sheet feeding apparatus comprises a cylindrical roller 1 rotatably supported by a body 2 and rotatingly controlled by a clutch CL and the like. An uppermost sheet P of a sheet stack contained in a sheet cassette 3 is urged against the roller 1 by means of a bias spring 5 via an imtermediate plate 4 of the cassette. Further, a rubber sheet 6 made of rubber material with cork embedded therein is pressed against the roller 1 at a downstream side of a contact area between the roller 1 and the sheet stack with respect to a sheet feeding direction. The rubber sheet 6 is fixed to a guide plate 7 which is rotatably supported by a shaft 7a extending perpendicular to the sheet feeding direction. The guide 7 is biased toward the roller 1 by means of a spring 8 so that the rubber sheet 6 is pressed against the roller 1 with a predetermined pressure. As sheets pass through a contact area between the rubber sheet 6 and the roller 1, the sheet P is separated one by one and is fed out.
However, in the above-mentioned conventional sheet feeding apparatus, as the sheet P is being fed pinched by the roller 1 with the rubber sheet 6 acting as the frictional separating means, fine vibration is created due to the sliding friction between the sheet P and the rubber sheet 6, which is transmitted to the rubber sheet 6 and the guide plate 7 supporting it, thus generating noise. The noise varies with the kind of sheets such as an OHP sheet, and, since such noise becomes the continuous discordant noise during the feeding of the sheet, in the conventional sheet feeding apparatus, such noise was minimized by increasing the weight of a supporting member for the guide plate 7, i.e., the separating means, or by providing a weight for preventing the vibration, or by reducing the coefficient of friction of the surface of the separating means. However, since the frictional separating means is urged against the roller 1 by the elastic means such as the spring 8 and the like, it was very difficult to suppress the vibration.
Further, when the sheet is fed to a next feeding means (for example, registering rollers) 9 in the frictional separating portion, since, in the separating portion, the sheet P is pinched between the rubber sheet 6 of the frictional separating means and the roller 1 which is now freely rotated due to an OFF condition of the clutch CL, the sheet P continues to slidingly contact with the rubber sheet 6 until a trailing end of the sheet leaves the rubber sheet. In addition, the rubber sheet 6 is so arranged that it can follow the movement of the sheet P by smoothly rocking or pivoting during the feeding of the sheet P. Thus, the fine vibration created by the sliding friction between the rubber sheet 6 and the sheet P is transmitted to the whole structure including the guide plate 7, thus creating significant noise.
Now, the generation of the fine vibration will be fully described.
When the sheet P is drawn between the rubber sheet 6 having high coefficient of friction and the freely-rotated roller 1, the sheet first remains stationary due to the static friction until a force applied to the sheet reaches a predetermined value. When the force exceeds the predetermined value, the sheet P starts to move. As the sheet P moves, the dynamic friction is gradually increased to decrease the speed of the sheet P, and, ultimately to stop the sheet again. When the sheet is stopped, since the static friction acts on the sheet P, the latter remains stationary until the force reaches the predetermined value. In this way, the movement and stop of the sheet P are repeated alternately (this is called as "stickslip"). These repeated movements are amplified by the pivotable guide plate 7 to create the great vibration in the sheet feeding direction, thus generating the noise.